1. Technical Field of the Invention
The present invention relates to analysis of analytes, such as metals, refractories, and slag, for the content of oxygen or oxide. More particularly, the present invention relates to a method and apparatus for analyzing an analyte for the content of oxygen or oxide for each oxide, wherein oxygen contained in an analyte is extracted as CO gas which is then measured by inert gas carrying/infrared absorption analysis.
2. Description of the Prior Art
In recent years, an analytical technique for accurately and rapidly analyzing an analyte for the content of oxygen or oxide has been desired in the art.
For example, in the field of steelmaking, the development of ultra low oxygen steel or high purity iron with the form of oxides being controlled has been progressed, and quantitatively determining the oxygen concentration on a very low level of ppm (parts per million) with high accuracy has been required. To this end, at the time of analyzing an analyte for a very small amount of oxygen, a contaminant, such as an oxide film, formed on the surface of the analyte should be removed. Electrolytic polishing or chemical polishing has been used for pretreatment at the time of analysis for oxygen wherein contaminants, such as an oxide film, are removed.
The electrolytic polishing is a method wherein contaminants, such as an oxide film, formed on the surface of an analyte is removed with a nonaqueous solvent electrolyte, such as a 10% acetylsalicyclic acid/1% tetramethylammonium chloride/methyl alcohol solution or a 4% sulfosalicyclic acid/1% lithium chloride/methyl alcohol solution. On the other hand, the chemical polishing is a method wherein an iron and steel analyte is immersed in a solution, such as hydrogen fluoride/hydrogen peroxide (HF-H.sub.2 O.sub.2), to remove a contaminant, such as an oxide film, formed on the surface of the analyte (for example, Hisao Yasuhara et al.: CAMP-ISIJ, 10(1997), p. 709).
However, the amount of oxygen present as an oxide on the surface of the analyte is not constant. For example, the amount of the surface oxide removed varies depending upon a polishing solution, a polishing time or the like, leading to a large variation in analytical value. Further, electrolytic polishing or chemical polishing of the surface of the analyte is disadvantageous in that the pretreatment of the analyte is troublesome and requires a lot of time.
In order to solve the above problem, a method for analyzing an iron and steel for a very small amount of oxygen has been proposed (Japanese Patent Laid-Open No. 148170/1994). This method comprises the steps of: grinding the surface of an iron and steel analyte by means of a grinder, a file or the like; heat-extracting a very small amount of oxygen contained in the analyte; and determining the extracted oxygen, wherein the analyte after the grinding is placed in a carbon crucible and preheated at a temperature of 900 to 1400.degree. C. to separate and determine oxygen derived from contaminants, such as deposited oxygen and an oxide film, on the surface of the analyte and oxygen in the iron and steel in the form of an oxide inclusion.
The analysis of an analyte for the content of oxygen by the above proposed method, however, has a problem that a point, where oxygen is evolved from an oxide inclusion (point D shown in FIG. 1), overlaps with the first wave, although the degree of overlapping varies depending upon the type, amount, and particle diameter distribution of oxide inclusions. This causes a part of the oxygen evolved from the oxide inclusion to be embraced in the amount of oxygen evolved from the surface deposited oxygen and iron oxide, making it impossible to accurately determine only the amount of oxygen evolved from the oxide inclusion.
On the other hand, in a bearing steel used under severe conditions, particularly Al.sub.2 O.sub.3, MgO.cndot.Al.sub.2 O.sub.3, and (Ca, Mg)O.cndot.Al.sub.2 O.sub.3 inclusions, among inclusions present in a very small amount, are likely to form large grains causative of fatigue failure. For this reason, a reduction in amount of inclusions in the product and the control of the form of inclusions are important, and a technique, which can accurately and rapidly analyze a low oxygen steel for each inclusion, has been desired in the art.
Conventional methods for analyzing a steel for inclusions include: a method which comprises extracting a specimen from an analyte, observing a test surface under a microscope, and classifying inclusions into A to C series (JIS G 0555); and a method wherein the surface of an analyte is subjected to mirror polishing and then instrumentally analyzed by electron beam microanalysis or the like.
In these methods, however, only a certain cross section of the analyte is analyzed. This poses problems including that true inclusions causative of material failure cannot be detected, the measuring time taken is very long, and pretreatment of the analyte, such as polishing, is troublesome.
An analytical method using an oxygen analyzer has recently been proposed as means for solving these problems (Japanese Patent Laid-Open No. 148167/1994). In this method, an analyte is placed in a graphite crucible and continuously heated at a constant temperature rise rate to separate oxygen derived from easily reducible oxides (such as FeO and MnO), which are decomposed on a relatively low temperature side, and oxygen derived from sparingly reducible oxides (such as CaO and Al.sub.2 O.sub.3), which are decomposed on a relatively high temperature side, from each other utilizing a CO gas extraction curve at the time of analysis.
The method described in Japanese Patent Laid-Open No. 148167/1994 is intended for a high content of oxygen of not less than 10% in steelmaking slag, and application of this method to a metal analyte containing oxygen in a very small amount on the order of several ppm has revealed that peaks in CO gas extraction curves are small and waves in respective CO gas extraction curves overlap with each other or one another, making it difficult to separate waves from each other or one another, that is, making it difficult to apply the above method to a metal analyte containing oxygen in a very small amount on the order of several ppm. Further, the above method is intended for separation of oxygen derived from easily reducible oxides and oxygen derived from sparingly reducible oxides and cannot be applied to analysis for oxygen in each oxide inclusion present in a very small amount.